This invention relates to a tachometer device controlled by an electric motor.
In particular, this invention relates to a tachometer device which enables rotary motion to be transmitted from a first to a second shaft by an electrical system, and enables the angular speed of said motion to be measured.
Known devices of the aforesaid type generally comprise a first and second electrical signal generator which at their outputs provide pulses which have a recurrence frequency proportional to the angular speed of the first and second said shaft respectively. In addition, by means of a processing circuit, such generators govern the operation of a preferably direct current electric motor, one shaft of which is torsionally coupled to the second aforesaid shaft.
The operating principle of these known devices consists of supplying the electric motor with a signal proportional to the difference between the angular speeds of the two shafts, as determined by the respective generators, so as to keep the speed of the second shaft substantially equal to the speed of the first.
An indicator and an integrator are connected to the second shaft in order to indicate respectively the angular speed of the first shaft and the number of revolutions made thereby. By using such devices on vehicles, the speed of movement and the distance travelled by the vehicle can be measured.
The advantage of a considerable noise reduction is obtained, because a flexible cable is not required for transmitting motion from the first to the second shaft. Furthermore, because the direct current electric motor has a high static torque, the second shaft is easily set into rotation, so preventing any undesirable crawling thereof.
However, these devices have certain drawbacks, which are particularly related to the practical embodiment of the described concept.
In particular, the processing circuit is complicated and easily subject to faults, because of which it requires constant checks and maintenance by expert personnel. Moreover, the various component parts (motor, second pulse generator, indicator and integrator) are often bulky to assemble, and large seats have to be made available to house them.